1. Field of the Invention
The present invention relates generally to an architecture for a Coded Signal Processing Engine (CSPE) that is designed for interference cancellation in the reception of coded signals. More particularly, the CSPE may be used for acquiring, tracking and demodulating pseudorandom (PN) coded signals in the presence of interference from other PN coded signals in a CDMA system.
2. Description of the Prior Art
In spread spectrum systems, whether it is a communication system, a Global Positioning System (GPS) or a radar system, each transmitter may be assigned a unique code and in many instances each transmission from a transmitter is assigned a unique code. The code is nothing more than a sequence (often pseudorandom) of bits. Examples of codes include the Gold codes (used in GPSxe2x80x94see Kaplan, Elliot D., Editor, Understanding GPS: Principles and Applications, Artech House 1996), Barker codes (used in radarxe2x80x94see Stimson, G. W., xe2x80x9cAn Introduction to Airborne Radarxe2x80x9d, SciTech Publishing Inc., 1998) and Walsh codes (used in communications systems like CDMAOnexe2x80x94See IS-95). These codes may be used to spread the signal so that the resulting signal occupies some specified range of frequencies in the electromagnetic spectrum or the codes may be superimposed on another signal which might also be a coded signal.
Assigning a unique code to each transmitter allows the receiver to distinguish between different transmitters. An example of a spread spectrum system that uses unique codes to distinguish between transmitters is a GPS system.
If a single transmitter has to broadcast different messages to different receivers, such as a base-station in a wireless communication system broadcasting to different mobiles, one may use codes to distinguish between the messages for each mobile. In this scenario, each bit for a particular user is encoded using the code assigned to that user. By coding in this manner, the receiver, by knowing its own code, may decipher the message intended for it from the composite signal transmitted by the transmitter.
In some communication systems, a symbol is assigned to a sequence of bits that constitute a message. For example, a long digital message may be grouped into sets of M bits and each one of these sets of M bits is a assigned to a symbol. For example, if M=6, then each set of 6 bits may assume one of 26=64 possibilities. One such possibility is 101101. Such a system would broadcast a unique waveform, called a symbol, to indicate to the receiver a particular sequence of bits. For example, the symbol xcex1 might denote the sequence 101101 and the symbol xcex2 might denote the sequence 110010. In the spread spectrum version of such a system, the symbols are codes. An example of such a communication system is the mobile to base-station link of CDMAOne.
In some instances, such as in a coded radar system, each pulse is assigned a unique code so that the receiver is able to distinguish between the different pulses based on the codes.
Of course, all of these techniques may be combined to distinguish between transmitters, messages, pulses and symbols all in one single system. The key idea in all of these coded systems is that the receiver knows the codes of the message intended for it and by applying the codes correctly to the received signal, the receiver may extract the message intended for it. However, such receivers are more complex than receivers that distinguish between messages by time and/or frequency alone. The complexity arises because the signal received by the receiver is a linear combination of all the coded signals present in the spectrum of interest at any given time. The receiver has to be able to extract the message intended for it from this linear combination of coded signals.
The following section presents the problem of interference in linear algebraic terms followed by a discussion of the current, generic (baseline) receiver.
Let H be a vector containing the spread signal from source number 1 and let xcex81 be the amplitude of the signal from this source. Let si be the spread signals for the remaining sources and let xcfx86i be the corresponding amplitudes. Suppose the receiver is interested in source number 1, the signals from the other sources may be considered to be interference. Then, the received signal is:
y=Hxcex81+s2xcfx862+s3xcfx863+ . . . +spxcfx86p+nxe2x80x83xe2x80x83(1)
where n is the additive noise term, and p is the number of sources in the CDMA system. Let the length of the vector y be N, where N is the number of points in the integration window. This number N is selected as part of the design process as part of the trade-off between processing gain and complexity. A window of N points of y will be referred to as a segment.
In a wireless communication system, the columns of the matrix H represent the various coded signals and the elements of the vector xcex8 are the powers of the coded signals. For example, in the base-station to mobile link of a CDMAOne system, the coded signals might be the various channels (pilot, paging, synchronization and traffic) and all their various multipath copies from different base-stations. In the mobile to base-station link, the columns of the matrix H might be the coded signals from the mobiles and their various multipath copies.
In a GPS system, the columns of the matrix H are the coded signals being broadcast by the GPS satellites at the appropriate code, phase and frequency offsets.
In an array application, the columns of the matrix are the steering vectors or equivalently the array pattern vectors. These vectors characterize the relative phase recorded by each antenna in the array as a function of the location and motion dynamics of the source as well as the arrangement of the antennas in the array. In the model presented above, each column of the matrix H signifies the steering vector of a particular source.
Equation (1) may now be written in the following matrix form:                                                         y              =                                                H                  ⁢                                      xe2x80x83                                    ⁢                  θ                                +                                  S                  ⁢                                      xe2x80x83                                    ⁢                  φ                                +                n                                                                                        =                                                                    [                    HS                    ]                                    ⁡                                      [                                                                                            θ                                                                                                                      φ                                                                                      ]                                                  +                n                                                                        (        2        )            
where
H: spread signal matrix of the source that the receiver is demodulating
xcex8: amplitude vector of the source that the receiver is demodulating
S=[s2 . . . sp]: spread signal matrix of all the other sources, i.e., the interference
xcfx86=[xcfx862 . . . xcfx86p]: interference amplitude vector
Receivers that are currently in use correlate the measurement, y, with a replica of H to determine if H is present in the measurement. If H is detected, then the receiver knows the bit-stream transmitted by source number 1. Mathematically, this correlation operation is:
correlation function=(HTH)xe2x88x921HTyxe2x80x83xe2x80x83(3)
where T is the transpose operation.
Substituting for y from equation (2) illustrates the source of the power control requirement:                                                                                                               (                                                                  H                        T                                            ⁢                      H                                        )                                                        -                    1                                                  ⁢                                  H                  T                                ⁢                y                            =                                                                    (                                                                  H                        T                                            ⁢                      H                                        )                                                        -                    1                                                  ⁢                                                      H                    T                                    ⁡                                      (                                                                  H                        ⁢                                                  xe2x80x83                                                ⁢                        θ                                            +                                              S                        ⁢                                                  xe2x80x83                                                ⁢                        φ                                            +                      n                                        )                                                                                                                          =                                                                                          (                                                                        H                          T                                                ⁢                        H                                            )                                                              -                      1                                                        ⁢                                      H                    T                                    ⁢                  H                  ⁢                                      xe2x80x83                                    ⁢                  θ                                +                                                                            (                                                                        H                          T                                                ⁢                        H                                            )                                                              -                      1                                                        ⁢                                      H                    T                                    ⁢                  S                  ⁢                                      xe2x80x83                                    ⁢                  φ                                +                                                                            (                                                                        H                          T                                                ⁢                        H                                            )                                                              -                      1                                                        ⁢                                      H                    T                                    ⁢                  n                                                                                                        =                              θ                +                                                                            (                                                                        H                          T                                                ⁢                        H                                            )                                                              -                      1                                                        ⁢                                      H                    T                                    ⁢                  S                  ⁢                                      xe2x80x83                                    ⁢                  φ                                +                                                                            (                                                                        H                          T                                                ⁢                        H                                            )                                                              -                      1                                                        ⁢                                      H                    T                                    ⁢                  n                                                                                        (        4        )            
It is the middle term, (HTH)xe2x88x921HTSxcfx86, in the above equation that results in the near-far problem. If the codes are orthogonal, then this term reduces to zero, which implies that the receiver has to detect xcex8 in the presence of noise (which is (HTH)xe2x88x921HTn) only. It is easy to see that as the amplitude of the other sources increase, then the term (HTH)xe2x88x921HTSxcfx86 contributes a significant amount to the correlation function, which makes the detection of xcex8 more difficult.
The normalized correlation function, (HTH)xe2x88x921HT, defined above, is in fact the matched filter and is based on an orthogonal projection of y onto the space spanned by H. When H and S are not orthogonal to each other, there is leakage of the components of S into the orthogonal projection of y onto H. This leakage is geometrically illustrated in FIG. 1. Note in FIG. 1, that if S were orthogonal to H, then the leakage component goes to zero as is evident from equation 4. The present application addresses a solution to this leakage issue.
It is therefore an object of the present invention to provide an adaptive interference canceller that addresses the near-far problem when S is not orthogonal to H.
It is a further object to provide a communication system that will allow for the mitigation of cross channel interference.
It is yet another object to provide a communication system that will allow for the mitigation of co-channel interference.
It is yet another object to provide a communication system that will allow for the mitigation of both cross channel interference and co-channel interference.
In all of the above embodiments, it is an object to provide a communication system that will increase the gain associated with a signal of interest in relation to co-channel and/or cross-channel interference.
In all of the above embodiments, it is an object to provide a communication system that mitigates interference without conducting iterative searches that involve matrix inversions and therefore reduce the mathematical complexity of the communications system.
Finally, it is an object of the invention to provide a method of interference mitigation that utilizes a projection method to effectively cancel interference without requiring knowledge of absolute power.
According to a first broad aspect of the present invention, there is provided a communication system having a forward link comprising: a base station which transmits multiple radio frequency (RF) signals; and at least one mobile station, the mobile station including: a receiver for receiving the RF signals; means for converting the RF signal to an intermediate frequency (IF) (including baseband) signal; means for sampling the IF analog signal to generate an IF digital signal, the digital signal having a data component and an interference component; means for canceling co-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal; and means for acquisition and tracking of the projected digital signal.
According to another broad aspect of the present invention, there is provided a mobile station, the mobile station for receiving at least two RF signals from the same source and comprising: a receiver for receiving the RF signals; means for converting the RF signal to an intermediate frequency (IF) (including baseband) signal; means for sampling the IF analog signal to generate an IF digital signal, the digital signal having a data component and an interference component; means for canceling co-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal; and means for acquisition and tracking of the projected digital signal.
According to another broad aspect of the present invention, there is provided a method for receiving signals in a receiver having at least one receiver circuit, the method comprising the steps of: receiving at least two RF signals from the same source; converting the RF signals to an intermediate frequency (IF) (including baseband) signals; sampling the IF analog signals to generate IF digital signals, the digital signal having a data component and an interference component; and canceling co-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal.
According to another broad aspect of the present invention, there is provided a communication system having a reverse link comprising: at least one mobile station which transmits radio frequency (RF) signals; and at least one base station, the base station including: a receiver for receiving at least two RF signals from the mobile station; means for converting the RF signals to an intermediate frequency (IF) (including baseband) signals; means for sampling the IF analog signals to generate IF digital signals, the digital signal having a data component and an interference component; means for canceling co-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal; and means for acquisition and tracking of the projected digital signal.
According to another broad aspect of the present invention, there is provided a base station, the base station for receiving at least two RF signals from the same source and comprising: a receiver for receiving the RF signals; means for converting the RF signals to a intermediate frequency (IF) (including baseband) signals; means for sampling the IF analog signals to generate IF digital signals, the digital signal having a data component and an interference component; means for canceling co-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal; means for acquisition and tracking of the projected digital signal.
According to another broad aspect of the present invention, there is provided a method for receiving signals in a receiver having at least one receiver circuit, the method comprising the steps of: receiving at least two RF signals broadcast from one mobile receiver; converting the RF signal to an intermediate frequency (IF) (including baseband) signal; sampling the IF analog signals to generate IF digital signals, the digital signal having a data component and an interference component; and canceling co-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal.
According to another broad aspect of the present invention, there is provided a communication system having a forward link comprising: at least one base station which transmits radio frequency (RF) signals; and at least one mobile station, the mobile station including: a receiver for receiving the RF signals; means for converting the RF signal to an intermediate frequency (IF) (including baseband) signal; means for sampling the IF analog signal to generate an IF digital signal, the digital signal having a data component and an interference component; means for canceling cross-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal; and means for acquisition and tracking of the projected digital signal.
According to another broad aspect of the present invention, there is provided a mobile station, the mobile station for receiving an RF signal and comprising: a receiver for receiving the RF signals; means for converting the RF signal to an intermediate frequency (IF) (including baseband) signal; means for sampling the IF signal to generate a digital signal, the digital signal having a data component and an interference component; means for canceling cross-channel interference in the digital signal by projecting the IF signal into a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal; and means for acquisition and tracking of the projected digital signal.
According to another broad aspect of the present invention, there is provided a method for receiving signals in a receiver having at least one receiver circuit, the method comprising the steps of: receiving at least one RF signal; converting the RF signal to an intermediate frequency (IF) (including baseband) signal; sampling the IF analog signal to generate a digital IF signal, the digital signal having a data component and an interference component; and canceling cross-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal.
According to another broad aspect of the present invention, there is provided a communication system having a reverse link comprising: at least one mobile station which transmits radio frequency (RF) signals; and at least one base station, the base station including: a receiver for receiving the RF signals; means for converting the RF signal to an intermediate frequency (IF) (including baseband) signal; means for sampling the IF analog signal to generate a digital IF signal, the digital signal having a data component and an interference component; means for canceling cross-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal; and means for acquisition and tracking of the projected digital signal.
According to another broad aspect of the present invention, there is provided a base station, the base station for receiving at least two RF signals and comprising: a receiver for receiving the RF signals; means for converting the RF signal to an intermediate frequency (IF) (including baseband) signal; means for sampling the IF analog signal to generate a digital IF signal, the digital signal having a data component and an interference component; means for canceling cross-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal; and means for acquisition and tracking of the projected digital signal.
According to another broad aspect of the present invention, there is provided a method for receiving signals in a receiver having at least one receiver circuit, the method comprising the steps of: receiving at least one RF signal broadcast from at least one mobile receiver; converting the RF signal to an intermediate frequency (IF) (including baseband) signal; sampling the IF analog signal to generate a digital IF signal, the digital signal having a data component and an interference component; and means for canceling cross-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal.
According to another broad aspect of the present invention, there is provided a communication system having a forward link comprising: at least one base station which transmits multiple radio frequency (RF) signals; and at least one mobile station, the mobile station including: a receiver for receiving the RF signals; means for converting the RF signal to an intermediate frequency (IF) (including baseband) signal; means for sampling the analog IF signal to generate a digital IF signal, the digital signal having a data component and an interference component; and means for canceling co-channel and cross-channel interference in the digital signal by projecting the digital signal onto a subspace orthogonal to a subspace of the interference component and multiplying this projection with the digital signal; and means for acquisition and tracking of the projected digital signal.
Other objects and features of the present invention will be apparent from the following detailed description of the preferred embodiment.